Easy way to convert text files to UTF-8 on Ubuntu. Install package enca and you are able to:
enconv -L pl -x UTF-8 myfile.txt
Where after -L is language specified (necessary for enca to recognize file encoding before conversion) and after -x destination encoding.
Warning: enconv overwrites existing file, so better create backup copy before.
UPDATE: Another way, useful if you know source file encoding:
iconv -f ISO-8859-2 -t UTF-8 source.txt > utf8.txt
